This invention relates to a length or angle measuring system of the type comprising a measuring graduation which defines a graduation axis extending along the graduation and a scanning unit positioned to scan the graduation, in which means are provided for coupling the graduation and the scanning unit to two relatively movable objects. In particular, this invention relates to an improved error correction system for such a measuring system.
A variety of error correction systems for such position measuring systems are known to the art.
For example, in U.S. Pat. No. 4,170,829 there is described a correction system for a length or angle measuring device in which a measuring graduation of the measuring device is slightly deflected at selected positions in a direction substantially perpendicular to the plane of the graduation by pressure or tension forces applied in correspondence to the desired error correction course. An array of adjusting members is mounted in the carrier body for deflecting the measuring graduation as desired. The accuracy of this nonlinear error correction system depends in part upon the number and spacing of the adjusting members per unit of measuring length.
U.S. Pat. No. 4,060,903 discloses a length measuring system in which a linear error correction is provided by means of longitudinal stretching or compression of the scale. Stretching and compression devices are provided at both ends of the measuring instrument to bring about the desired change in dimension of the scale. This correction system does not permit nonlinear error correction.
U.S. Pat. No. 4,170,828 describes a length measuring system which incorporates an error correction system which comprises a link chain. Individual members of this chain are adjustable in accordance with the desired error correction course transversely to the measuring direction. A transfer element is positioned to scan this link chain and to bring about a correcting relative movement along the measuring direction between a scanning unit (which is guided in parallel motion with respect to the graduation plane of the scale) and the scale. In this error correction system, the accuracy of the error correction is dependent upon the number of chain link members provided per unit of measuring length.
U.S. Pat. No. 4,262,423 discloses an error correction system for a length measuring instrument, in which an error correction profile is formed as an integral component of a carrier or housing for the scale. This error correction profile is scanned by the transfer element which brings about a correcting relative movement along the measuring direction between the scanning unit and the scale. The scanning unit is guided in parallel motion with respect to the graduation plane of the scale.
In the two last-mentioned correction systems, the transfer elements take the form of pivotable angle elements which are subject to mechanical wear. Such pivotable angle elements can bring about a substantial increase in the cross-sectional dimensions of the position measuring system. Such an increase in the size of the position measuring system can be detrimental to a flexible use of the measuring system in certain applications.